Time-dependent trellis coding for more robust digital television signals

ABSTRACT

Different sets symbols are precluded at prescribed times in time-dependent trellis coding. This increases the distances between different individual symbols as well as the distances between trellis codes, which increases the robustness of data transmission. The symbols that are precluded in this time-dependent trellis coding are determined in advance according to a prescribed pattern, which pattern does not depend on the history of previous symbols. The Viterbi decoder used for trellis decoding in a receiver can be designed to take advantage of knowledge concerning which different sets of symbols are precluded at prescribed times.

This application, filed under 35 U.S.C. 119(e)(1) benefit of the filingdates of provisional U.S. patent applications Ser. Nos; 60/507,797,60/524,984 and 60/531,124 filed under 35 U.S.C. 111(b) on 1 Oct. 2003,25 Nov. 2003 and 19 Dec. 2003, respectively. These provisional patentapplications in their entirety incorporated by reference herein.

This invention relates to symbol coding of digital signals such as thoseused for broadcasting digital television.

BACKGROUND OF THE INVENTION

The MPEG-2 standard addresses the combining of one or more elementarystreams of video audio and other data into single or multiple streamsthat are suitable for storage or transmission. In very general terms,the MPEG-2 standard for transmitting digital video, the associated audioand other information involves the following three steps. In the firststep, a digital video signal (from a digital camera or from an analog todigital converter) is compressed by analyzing and encoding the signalusing spatial and temporal redundancy. Spatial redundancy refers to theredundant information inside one video frame while temporal redundancyrefers to the redundant information between consecutive frames. Thisprocess generates: Intra-frames (I-frames), which contain all of theinformation in an entire image; Predicted frames (P-Frames), which havesome compression as they are predicted based on past I-frames and/orother P-frames; and Bi-directionally predicted frames (B-frames), whichare the most compressed images as they are predicted from past andfuture I-Frames and P-Frames. In the second step carried outconcurrently with the first step, an audio signal is compressed byremoving low-power tones adjacent high-power tones. Removal of thesetones does not affect the signal, because the high-power tones tend tomask the lower-power tones, making them inaudible to the human ear. Inthe final third step, the compressed, video signals audio signals andrelated time stamps of those signals are assembled into packets andinserted into a Packetized Elementary Stream (PES). Each packet in apacketized elementary stream contains overhead information such as astart code, stream ID, packet length, optional packetized elementarystream header and stuffing bytes, in addition to the actual packet bytesof video and audio data.

To facilitate the multiplexing together of several streams of packetizedelementary streams of different types of data, a Programme SpecificInformation (PSI) table is also created, which includes a series oftables to reassemble specific packetized elementary stream withinmultiple channels of packetized elementary streams. The packetizedelementary stream and the program specific information provide the basisfor a Transport Stream (TS) of packetized elementary stream and programspecific information packets.

Of particular interest to the invention disclosed herein is thetransport stream as defined in Annex D of the “ATSC Digital TelevisionStandard” published by the Advanced Television Systems Committee ATSC)in September 1995 as its document A/53. This standard defines thebroadcasting of digital television (DTV) signals within the UnitedStates of America and is referred to in this specification simply as“A/53”. Annex D specifies that the original data transport stream iscomposed of 187-byte packets of data corresponding to MPEG-2 packetswithout their initial sync bytes. Annex D specifies that data are to berandomized by being exclusive-ORed the a specific 2¹⁶-bit maximal lengthpseudo-random binary sequence (PRBS) which is initialized at thebeginning of each data field. Annex D specifies (207, 187) Reed-Solomonforward-error-correction (R-S FEC) coding of packets of randomized datafollowed by convolutional interleaving. The convolutional interleavingprescribed by A/53 provides error correction capability for continuousburst noise up to 193 microseconds (2070 symbol epochs) in duration. Theconvolutionally interleaved data with R-S FEC coding are subsequentlytrellis coded to ⅔ original code rate and mapped into eight-leveldigital symbols. The symbols are parsed into 828-symbol sequences.

Annex D specifies that the data frame shall be composed of two datafields, each data field composed of 313 data segments, and each datasegment composed of 832 symbols. Annex D specifies that each datasegment shall begin with a 4-symbol data-segment-synchronization (DSS)sequence. Annex D specifies that the initial data segment of each datafield shall contain a data-field-synchronization (DFS) signal followingthe 4-symbol DSS sequence therein. The DSS and DFS signals are composedof symbols with +5 or −5 modulation signal values. The 2^(nd) through313^(th) data segments each conclude with a respective one of thetrellis-coded 828-symbol sequences, the convolutional interleaving ofwhich sequences extends to a depth of 52 data segments. The digitalsymbols are transmitted by eight-level modulation with +7, +5, +3, +1,−1, −3, −5 and −7 modulation signal values. Owing to the A/53 basebandDTV signal being transmitted via vestigial-sideband suppressed-carrieramplitude modulation of a radio-frequency carrier, this eight-levelmodulation signal is referred to as trellis-coded 8VSB signal. Thesetransmissions are accompanied by a pilot carrier of the same frequencyas the suppressed carrier and of an amplitude corresponding tomodulation value of +1.25.

The fifth through 515^(th) symbols in the initial data segment of eachdata field are a specified PN511 sequence—i. e., a pseudo-random noisesequence composed of 511 symbols capable of being rendered as +5 or −5modulation signal values. The 516^(th) through 704^(th) symbols in theinitial data segment of each data field are a triple-PN63 sequence. Themiddle PN63 sequence is inverted in sense of polarity every other datafield. The 705^(th) through 728^(th) symbols in the initial data segmentof each data field contain a VSB mode code specifying the nature of thevestigial-sideband (VSB) signal being transmitted. The remaining 104symbols in the initial data segment of each data field are reserved,with the last twelve of these symbols being a precode signal thatrepeats the last twelve symbols of the data in the last data segment ofthe previous data field. A/53 specifies such precode signal to implementtrellis coding and decoding procedures being resumed in the second datasegment of each field proceeding from where those procedures left offprocessing the data in the preceding data field.

The 8VSB transmissions have a 10.76 million bits per second baud rate toit within a 6-megahertz-wide broadcast television channel, and theeffective payload is 19.3 million bits per second (Mbps). In anadditive-white Gaussian noise (AWGN) channel a perfect receiver willrequire at least a 14.9 dB signal-to-nose ratio (SNR) in order to keeperrors below a threshold of visibility (TOV) defined as 1.93 datasegment errors per 10,000 data segments, supposing 8VSB signals arebroadcast.

After the “ATSC Digital Television Standard” was established in 1995,reception of terrestrial broadcast DTV signals proved to be problematic,particularly if indoor antennas were used. In early 2000 ATSC made anindustry-wide call for experts in terrestrial broadcast transmission andreception to join a Task Force on RF System Performance for studyingproblems with adequate reception and suggesting possible solutions tothose problems. By the end of 2000 or so there was general consensusthat, besides problems with equalization of the reception channel, therewas a need to make the 8VSB signal more robust, if it were to besuccessfully received during noise reception conditions. On 26 Jan. 2001the ATSC Specialist Group on RF Transmission (T3/S9) issued a “Requestfor Proposal for Potential Revisions to ATSC Standards in the Area ofTransmission Specifications”. This RFP concerning how to improve theperformance of 8VSB was directed to the DTV industry, universities andother parties interested in the problem. The widely distributed T3/S9RFP specifies backward-compatible improvement of fixed and indoor 8-VSBterrestrial DTV service to be of top priority. The requirement forbackward-compatibility with legacy DTV receivers means, among otherthings, that the trellis coding specified in A/53 must be maintainedthroughout data fields.

A general approach to making 8VSB signal more robust is to increase theamount of forward-error-correction-coding. Zenith Electronics and ATIproposed the application of preliminary additional trellis coding todata before the trellis coding specified by A/53. Legacy DTV receiversalready in the field are incapable of receiving the data with theadditional trellis coding, however.

The general concept that FEC coding can be contained in data packetsthat do not contain payload data and that are separate from data packetsthat do contain payload is found in U.S. Pat. No. 6,430,159 thespecification and drawing of which are incorporated herein by reference.U.S. Pat. No. 6,430,159 titled “Forward error correction at MPEG-2transport stream layer” issued Aug. 6, 2002 to Xiang Wan and Marc H.Morin. Wan and Morin sought to provide a system and method to correct anMPEG-2 transport stream that could be used in any one of the digitalvideo broadcast formats, without the need for FEC decoders which werespecific to the particular format. Another objective of the U.S. Pat.No. 6,430,159 invention was to avoid appending FEC coding to the end ofeach packet, in effect adding another layer to the protocol stack. Sucha new layer is specific to the transmission architecture and not subjectto the MPEG-2 standard, so a broadcaster has to rely upon each intendedreceiver having a symmetric FEC decoder for the transmitted signal to bereceived. However, the satellite broadcast industry the cablecastingindustry and the terrestrial broadcast industry embraced the practice ofinserting the original transport stream into a forward-error-correctionencoder and broadcasting the resulting signal over their respectivebroadcast medium to receivers. The various receivers for satellitebroadcast, cablecasting and terrestrial broadcast systems recoverMPEG-2-compliant transport streams from received signals, usingFEC-decoders specific to the various systems and symmetric with the FECencoders employed in these various systems.

A. L. R. Limberg ran across U.S. Pat. No. 6,430,159 during acomprehensive review of DTV receiver practice he conducted in 2002 whenworking on the revision of the “Guide to the Use of the ATSC DigitalTelevision Standard” published in October 1995 as ATSC Document A/54.Limberg perceived that the Wan and Morin concept still had practicalutility, even though (207, 187) R-S FEC coding was appended to datasegments of 8VSB DTV broadcast signal, employing the sort of practiceWan and Morin had sought to avoid by their invention. Limberg perceivedthat transverse Reed-Solomon forward-error-correction coding facilitatesadditional error-correction coding being time-division multiplexed withA/53 data segments in such a way that DTV receivers already in the fieldcan still receive the A53 data segments. Limberg understood that the Wanand Morin concept was the key to solving the problem of making the DTVsignals more robust without making those signals impossible to bereceived by DTV receivers already in the field. This was the problemthat had stumped experts in DTV system design for two years or moredespite T3/S9 having focused industry effort on solving this problem.Limberg discerned that transverse R-S FEC coding was orthogonal to“lateral” (207, 187) R-S FEC coding prescribed by A/53 and combinedtherewith to provide two-dimensional R-S FEC coding.

An alternative approach to making 8VSB signal more robust is to restrictthe symbol alphabet in such a way that symbol decoding procedures areless susceptible of error. For example, a set of limited-alphabet 8VSBsymbols that map data into just +7, +5, 5 and −7 modulation signalvalues was proposed by Philips Research responsive to the T3/S9 RFP.This limited-alphabet signal is referred to as “pseudo-2VSB” or“P-2VSB”, since the polarity of the signal suffices to convey theinformation in the resulting modulation signal. Using P-2VSB throughoutthe entire DTV broadcast halves the effective payload to 9.64 millionbits per second (Mbps), but this is more than sufficient to transmit astandard-definition television (SDTV) signal. The gap between the leastnegative normalized modulation level, −5, and the least positivenormalized modulation level, +5, is 10. This is five times the gap of 2between adjacent normalized modulation levels in an 8VSB signal. The8VSB signal has ⅔ trellis coding, however, which increases itsperformance capability to be somewhat better than a VSB signal with agap of 4 between adjacent normalized modulation levels. Accordingly, theSNR required in order to keep error below TOV in an AWGN channel isreduced to 8.5 dB, a reduction of 6.4 dB. That is, about a quarter asmuch power would be required for satisfactory reception of an AWGNchannel, presuming that modulation levels did not have to be decreasedto maintain effective radiated power (ERP) levels within currentspecification. The ERP of the P-2VSB symbols tends to increaserespective to conventional trellis-coded 8VSB, because of just the +7,+5, −5 and −7 modulation signal values being used and the +3, +1, −1 and−3 modulation signal values of 8VSB not being used. A 1.5 dB decrease intransmitter ERP is necessary if long sequences of P-2VSB symbols aretransmitted. So, if long sequences of P-2VSB symbols are transmitted,increase in service area for the P-2VSB signal is only that which couldbe achieved with a 4.9 dB increase in the power of a conventionaltrellis-coded 8VSB signal. Furthermore, service area for theconventional trellis-coded 8VSB signal accompanying the P-2VSB signal isdiminished. Consequently, P-2VSB sign as shave been considered only foronly a limited number of the data segments in each 313 segment datafield.

The Electronics and Telecommunications Research Institute (ETRI) andChonnnam National University (CNU) Multimedia Communications Laboratoryin South Korea proposed another set of restricted-alphabet 8VSB subjectsthat map data into just +7, +1, −3 and −5 normalized modulation signalvalues. This type of signal is referred to as “enhanced-4VSB” or“E-4VSB”. The gap between the least negative normalized modulationlevel, −3, and the least positive normalized modulation level, +1, is 4.This is twice the gap of 2 between adjacent modulation levels in an 8VSBsignal, permitting TOV under AWGN conditions to be achieved atsignificantly poorer SNR than is the case with 8VSB signal. The SNR thatE-4VSB requires to keep errors below TOV is higher than that requiredwith pseudo-2VSB modulation.

The ERP of the E-4VSB symbols is purportedly the same as that ofconventional trellis-coded 8VSB. Consequently, considerably more datasegments in data fields can code E-4VSB modulation than can codepseudo-2VSB signals, presuming that ERP is not to be increased verymuch. E-4VSB is presumably less likely than pseudo-2VSB to disrupt theoperation of legacy receivers, particularly those that rely on symbolaveraging to develop automatic gain control signals for controlling thegains of their amplifier stages. The limitation on the number of thedata segments in each 313-segment data field that can be E-4VSB signalsdepends solely on the number of data segments of normal transmissionsthat must be provided to accommodate legacy receivers. The moreasymmetrical symbol constellation benefits symbol synchronization usingbright-spectral-line techniques.

Certain modifications of the original data transport stream cause thetrellis coding procedure at the transmitter to generate 8VSB symbolswith various restrictions of the available symbol alphabet. Each bit ina stream of randomized data can be repeated to generate a modifiedstream of data supplied to the (207, 187) R-S FEC encoder for example,to cause a pseudo-2VSB signal to be generated by the trellis codingprocedure. In other procedures for restricting the symbol alphabet foreach symbol epoch, each bit in a stream of randomized data can befollowed by an additional bit of prescribed value independent of the bitit follows.

By way of example, ONE can be inserted after each bit in a stream ofrandomized data to generate a modified stream of data supplied to the(207, 187) R-S FEC encoder. This modified stream of data causes thetrellis coding procedure to generate a restricted-alphabet signal whichexcludes the −7, −5, +1 and +3 symbol values of the full 8VSB alphabet.Pilot carrier energy is increased substantially in the resultingmodulation, which makes synchronous demodulation easier in the DTVreceiver. The gap between the least negative normalized modulationlevel, −5, and the least positive-normalized modulation level, +1 is 6in this restricted-alphabet signal. This gap is three times the gap of 2between adjacent modulation levels in an 8VSB signal, permitting TOV tobe achieved at significantly poorer SNR under AWGN conditions than isthe case with 8VSB signal or with E-4VSB signal. Better SNR under AWGNconditions is required to achieve TOV than is the case with pseudo-2VSB.This restricted-alphabet signal has substantially less average powerthan a pseudo-2VSB signal, but somewhat higher average power than normal8VSB signal.

By way of counter example, a ZERO can be inserted after each bit in astream of randomized data to generate a modified stream of data suppliedto the (207, 187) R-S FEC encoder. This modified stream of data causesthe trellis coding procedure to generate a restricted-alphabet signalwhich excludes the −3, −1, +5 and +7 symbol values of the full 8VSBalphabet. The gap between the least negative normalized modulationlevel, −5, and the least positive normalized modulation level, +1, isalso 6 in this restricted-alphabet signal. However, thisrestricted-alphabet signal has somewhat less average power than normal8VSB signal. A difficult problem with using this restricted-alphabetsignal is that the polarity of the pilot signal is reversed in theresulting modulation, which interferes with synchronous demodulation inDTV receivers, particularly legacy ones.

The inventor subsequently discerned that the 8VSB alphabet can berestricted in such a way that, in accordance with a prescribed pattern,a ZERO or a ONE is inserted as an X₁ bit after each of the X₂ bits in adata segment to be incorporated into a data field for randomization, R-SFEC coding, convolutional interleaving, and trellis coding. If ZEROesand ONEs occur with similar frequency in the pattern, ERP can be keptsubstantially the same as in an ordinary 8VSB signal described in AnnexD of A/53. This requires careful selection of the prescribed pattern ofinserting ZEROes and ONEs as X₁ bits. If symbols are correctly sampled,the gap between the least negative normalized modulation level and theleast positive normalized modulation level is 6 in each symbol of thisrestricted-alphabet signal. This general type of restricted-alphabetsignal, constructed from co-sets of a complete symbol alphabet thatoccur at prescribed times, is an important aspect of certain of theinventions described in this specification. This general type ofrestricted-alphabet signal is also useful in applications other than8VSB DTV broadcasting, being useful in MPSK transmissions by way ofexample.

Viewed another way, this aspect of the invention concerns time-dependenttrellis coding in which different sets of symbols are precluded atprescribed times in order to increase the Hamming distances betweenpossible trellis codes. The symbols that are precluded are determined inadvance according to a prescribed pattern, which pattern does not dependon the history of previous symbols. The pattern can be chosen to adjustthe ERP of a transmitter such that average power is substantially thesame as for symbol coding in which symbols are randomly selected fromthe full 8VSB symbol alphabet. This time-dependent trellis codingdiffers from extended trellis coding in which the symbols that areprecluded are determined depending on the history of previous symbols.This time-dependent trellis coding is not subject to the tendencytowards running error in the decoding of trellis code increasing as thecode is extended. Each successive symbol in the time-dependent trelliscode exhibits increased Euclidean distance respective to other symbolsthat could occur during that symbol epoch, so the possibility of errorin hard-decision decoding is substantially reduced. This can be used forimproving adaptive equalizer convergence during rapidly changingmultipath conditions.

U.S. patent application Ser. No. 10/733,645 filed 12 Dec. 2003 for A, L.R. Limberg titled “Robust Signal Transmissions Digital TelevisionBroadcasting” describes transverse Reed-Solomon forward-error-correctioncoding being used to supplement the error correction coding already inthe 8VSB data segments. The parity bytes for the transverse Reed-Solomonforward-error-correction coding are arranged in A/53-compliant datasegments to be time-division multiplexed with conventional A/53 datasegments. The resulting signal is then convolutionally interleaved,trellis coded and mapped into 8VSB symbols per subsections 4.2.4 and4.2.5 of A/53, Annex D. Patent application Ser. No. 10/733,645specifically considers how transverse Reed-Solomonforward-error-correction coding of restricted-alphabet signals can bedone. Patent application Ser. No. 10/733,645 discloses a problem that isencountered when one attempts to apply transverse Reed-Solomonforward-error-correction coding to restricted-alphabet signals in whichthe Z₁ bit in a symbol codeword elected for the restricted-alphabetsignal cannot be determined independently of the Z₀ term. Suppose theparity bytes of the transverse R-S FEC coding were permitted tointerleave convolutionally with bytes of such a restricted-alphabetsignal. Then, the Z₀ bits in the symbol codewords of such arestricted-alphabet signal would depend on the Z₁ bits in the symbolcodewords of the parity bytes of the transverse R-S FEC coding. However,the Z₁ bits in the symbol codewords of the transverse R-S FEC codingshould depend or the Z₁ bits of the symbol codewords in therestricted-alphabet signal. This is a situation of trying to “liftoneself by one's own bootstraps”. E-4VSB signal has 001, 010, 100 and111 symbol codewords that respectively generate −5, −3, +1 and +7normalized modulation signal values. The Z₁ bits in the E-4VSB symbolcodewords cannot be determined independently of the Z₀ bits, so theE-4VSB signal does not lend itself to transverse R-S FEC coding, atleast not readily. Accordingly, there is a need for a type of robustmodulation that halves code rate without affecting average ERP, but alsobetter lends itself to transverse R-S FEC coding.

The known types of robust modulation that have code rate, but also lendthemselves to transverse R-S FEC coding, are ones with a set of foursymbol codewords for which the Z₁ bits can be determined independentlyof the Z₀ bits. The Z₁ bit repeats the Z₂ bit in all 3-bit symbolcodewords of pseudo-2VSB signals, so pseudo-2VSB modulation lends itselfto transverse R-S FEC coding. So does robust modulation whereinaccordance with a prescribed pattern a ZERO or a ONE is inserted as arespective Z₁ bit after the Z₂ bit in each 3 bit-symbol codeword beforeit is supplied to a trellis encoder.

A previous practice when including robust transmissions DTV signals hasbeen to confine the robust transmissions to the 184-byte payloadportions of data segments. Each data segment containing robusttransmission begins with a 3-byte header that causes the data segment tobe discarded by legacy 8VSB DTV receivers. Each data segment containingrobust transmission concludes with twenty parity bytes of R-S FECcoding. MPEG-2 data packets do not map to an integral number of datasegments when such previous practice is followed. Accordingly, suchprevious practice requires rather elaborate procedures for parsing datapackets, especially since data segments associated with robusttransmission have to be time-division multiplexed with data segmentsassociated with ordinary HDTV transmission. The procedures for parsingdata packets are apt to error during noisy reception.

The inventor prefers a new practice for including robust transmissionsin DTV signals. In this preferred practice a data segment containing a187-byte MPEG-2-compliant data packet and twenty bytes of lateral R-SFEC coding is converted into an integral number of consecutive datasegments, such as two, which provides for simple parsing. Theconsecutive data segments generated by this simple conversion procedurewill not be A/53 compliant, but this need not be problematic. Legacy DTVreceivers are incapable of usefully receiving restricted-alphabetcomponents of an 8VSB DTV broadcast signal anyway. Accordingly, the datasegments including robust transmissions are freed from having to meetcertain requirements, insofar as accommodating legacy DTV receives is ofconcern. These data segments do not each need to include a data packetcomplying with MPEG-2, and these data segments do not each need toinclude parity bytes of “lateral” (207, 187) R-S FEC coding asprescribed by A/53. These data segments should be ones that legacy DTVreceivers will discard during transport stream de-multiplexing, eitherbecause they do not appear to include recognizable PID code or becausethey are found not to be correctable during R-S FEC decoding procedures.Each data packet that is to be transmitted using a restricted symbolalphabet can be evaluated ahead of time. The evaluations are made toascertain which data randomization sequences would result in a legacyreceiver finding one or both of the data segments derived from that datapacket to contain both a valid PID and correctable byte errors.Transmission of the robust-data packet is scheduled in the data field sothat each portion of that packet in a respective data segment uses adata randomization sequence that results in byte errors beyond thecapability of correction by a standard (207, 187) R-S FEC decoder.

U.S. patent application Ser. 10/885,460 filed 6 Jul. 2004 for A. L. R.Limberg and titled “Reed-Solomon Coding Modifications for SignalingTransmission of Different Types of Data Packets” is incorporated byreference in this application. U.S. patent application Ser. No.10/885,460 discloses an alternative way to cause the robust-datasegments to contain byte errors beyond the capability of correction by astandard (207, 187) R-S FEC decoder. Each segment of robust data thatcontains byte errors within the capability of correction by a standard(207, 187) R-S FEC decoder is modified before transmission so this is nolonger the case. The modification causes shortened R-S coding that isdifferent than normal, so a legacy DTV receiver will find therobust-data segment to contain byte errors beyond the capability ofcorrection by its (207, 187) R-S FEC decoder. A new DTV receiver willundo this modification responsive to a byte errors in a data segmentbeing found to be correctable by a (207, 187) R-S FEC decoder for theshortened coding that is different than normal.

A DTV receiver that is adapted for useful receiving both full-alphabetand restricted-alphabet components of an 8VSB DTV broadcast signal hasto have knowledge of when each of these components is being received.This knowledge permits symbol decoding of the restricted-alphabetcomponents to be done in special way that improves the accuracy ofsymbol decoding decisions. The general procedure in the prior art is forthe DTV transmitter to transmit information to the DTV receiverconcerning the pattern of data segments recovered fromrestricted-alphabet components of the 8VSB DTV broadcast signal, whichpattern obtains in each data field before being convolutionallyinterleaved and trellis coded. This information is transmitted in thereserved portion of the initial data segments of data fields, variouscoding schemes for such information being known. U.S. Pat. No. 6,563,436titled “KERDOCK CODING AND DECODING SYSTEM FOR MAP DATA” and issued 13May 2003 to M. Fimoff, R. W. Citta and J. Xia describes one way of doingthis, for example. The pattern information is convolutionallyinterleaved to generate information concerning which symbols of theconvolutionally interleaved data field received by the DTV receiver areselected from a restricted alphabet of 8VSB symbols. Certain of the datasegments in the de-interleaved field that the de-interleaver generatesfrom trellis coding results are recovered from restricted-alphabetcomponents of the 8VSB DTV broadcast signal. The pattern informationavailable to a TV receiver is used in an additional way in novel DTVreceivers described in this specification and its drawing. The patterninformation is used to select these data segments for the datacompression that converts them to a reduced number of data segments thatcomply with A/53 standards for data segments recovered fromfull-alphabet components of the 8VSB DTV broadcast signal.

SUMMARY OF THE INVENTION

The invention in various of its aspects concerns time-dependent trelliscoding in which different sets symbols are precluded at prescribed timesin order to increase distances between different individual symbols aswell as the distances between trellis codes, which increases therobustness of data transmission. The symbols that are precluded aredetermined in advance according to a prescribed pattern, which patterndoes not depend on the history of previous symbols.

An aspect of the invention is restricting the symbol alphabet of adigital television signal by inserting, in accordance with a prescribedpattern, a ZERO or a ONE after each bit in a data segment to beincorporated into a data field for randomization, R-S FEC coding,convolutional interleaving, and trellis coding. Transmitters forbroadcasting DTV signals with the data segments so modified embodycertain aspects of the invention. Receivers for receiving those DTVsignals and reproducing data packets embody other aspects of theinvention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a generic schematic diagram of a DTV transmitter-constructedin accordance with an aspect of the invention.

FIG. 2 is a schematic diagram showing one particular construction of theX₁ bits generator for the FIG. 1 DTV transmitter.

FIG. 3 is a table showing a possible set of X₁ bits stored in read-onlymemory included in the X₁ bits generator of FIG. 2.

FIG. 4 is a schematic diagram of a modification that can be made to theDTV transmitter of FIG. 1, 6, 7 or 8 to provide for the transmission of8VSB and pseudo-2VSB signals in time-division multiplex.

FIG. 5 is a schematic diagram of a modification that can be made to theDTV transmitter of FIG. 1, 6, 7 or 8 to provide for the transmission of8VSB and enhanced-4VSB signals in time-division multiplex.

FIG. 6 is a schematic diagram of a modification of the FIG. 1 DTVtransmitter that in accordance with an aspect of the invention providestransverse Reed-Solomon forward-error-correction coding to data for anancillary service transmitted using a restricted alphabet of 8VSBsymbols.

FIG. 7 is a schematic diagram of another modification of the FIG. 1 DTVtransmitter, which modification provides transverse Reed-Solomonforward-error-correction coding to data transmitted using the fullalphabet of 8VSB symbols.

FIG. 8 is a schematic diagram of an alternative modification of the FIG.1 DTV transmitter, which modification provides transverse R-S FEC codingboth to data subsequently transmitted using the full alphabet of 8VSBsymbols and to data subsequently transmitted using a restricted alphabetof 8VSB symbols.

FIGS. 9A, 9B and 9C combine to form a FIG. 9 schematic diagram of a DTVreceiver capable of receiving DTV signals as transmitted by the FIG. 1DTV transmitter or modifications of that transmitter per any of FIGS. 4,5, 6, 7 and 8.

FIG. 10 is a listing of the steps in a routine to validate the insertionof restricted-alphabet data segments into the time-division multiplex(TDM) signal that defines a data field before subsequent convolutionalinterleaving and trellis coding carried out in a DTV transmitter of FIG.1 sort.

FIG. 11 is a listing of the steps in a routine to validate the insertionof restricted-alphabet data segments into the time-division multiplex(TDM) signal that defines a data field before subsequent convolutionalinterleaving and trellis coding carried out in a DTV transmitter of FIG.1 sort modified per FIG. 4.

FIG. 12 is a listing of the steps in a routine to validate the insertionof restricted-alphabet data segments into the time-division multiplex(TDM) signal that defines a data field before subsequent convolutionalinterleaving and trellis coding carried out in a DTV transmitter of FIG.6 sort.

FIG. 13 is a listing of the steps in a routine to validate the insertionof restricted-alphabet data segments into the time-division multiplex(TDM) signal that defines a data field before subsequent convolutionalinterleaving and trellis coding carried out in a DTV transmitter of FIG.6 sort modified per FIG. 4.

DETAILED DESCRIPTION

FIG. 1 shows a program source 1 of a principal transport streamconnected for supplying the successive 187-byte data packets in thattransport stream to be written into a first-in/first-out buffer memory 2for temporary storage therein. A data randomizer 3 is connected forreceiving data packets read from the FIFO buffer memory 2 andrandomizing the bits in those data packets by exclusive-ORing those bitswith the bits of a 2 ¹⁶-bit maximal length pseudo-random binary sequence(PRBS). The PRBS, which is initialized at the beginning of the each datafield, is that specified in A/53, Annex D, §§ 4.2.2 titled “Datarandomizer”. The portion of the PRBS used in exclusive-ORing each datasegment is that portion which is suitable for the location of that datasegment in the non-interleaved data field that will be assembled forsubsequent lateral (207, 187) R-S FEC coding convolutional interleavingand trellis coding. A lateral (207, 187) Reed-Solomonforward-error-correction encoder 4 is connected for receiving from thedata randomizer 3 the randomized 187-byte data packets of the principaltransport stream. The lateral (207, 187) R-S FEC encoder 4 convertsthese randomized 187-byte data packets to respective 207-byte segmentsof lateral (207, 187) Reed-Solomon forward-error-correction coding thatappends the respective twenty parity bytes of the coding of eachrandomized 187-byte data packet after the conclusion thereof. Thelateral (207, 187) R-S FEC encoder 4 is of a first type, which isconventional in nature; and the practice specified in A/53, Annex-D, §§4.2.3 titled “Reed-Solomon encoder” is followed. A time-divisionmultiplexer 5 used to assemble-data-fields is connected for receiving ata first of its two input ports the 207-byte segments of lateral (207,187) R-S FEC coding generated by the lateral (207, 187) R-S FEC encoder4.

FIG. 1 shows a program source 6 of an ancillary transport streamconnected for supplying the successive 187-byte data packets in thattransport stream to be written into a first-in/first-out buffer memory 7for temporary storage therein. A data randomizer 8 is connected forreceiving data packets read from the FIFO buffer memory 7. The datarandomizer 8 is operated for randomizing the bits in those data packetsby exclusive-ORing them with the bits of the PRBS advanced 1496 bits (1data packet) respective to the location of that data segment in thenon-interleaved data field. I.e., the portion of the PRBS used for PRBSexclusive-ORing these bits is suitable for the location of the next datasegment in the non-interleaved data field. This next data segment can bethought of as a null data segment that is replaced during a subsequentre-sampling procedure for halving code rate. A lateral (207, 187) ReedSolomon forward-error-correction encoder 8 of conventional first type isconnected for receiving from the data randomizer 7 the randomized 187byte data packets of the ancillary transport stream. The lateral (207,187) R-S FEC encoder 8 converts these randomized 187-byte data packetsto respective 207-byte segments of lateral (207, 187) R-S FEC codingthat appends the respective twenty parity bytes of the coding of eachrandomized 187-byte data packet after the conclusion thereof. Are-sampler 10 is connected for receiving these 207-byte segments fromthe lateral (207, 187) R-S FEC encoder 8 and generates in response toeach of these 207-byte segments a respective pair of 207-byte segmentsat halved code rate. The re-sampler 10 treats each of these 207-bytesegments from the lateral (207, 187) R-S FEC encoder 8 as consisting ofthe X₂ bits utilized in the data stream, that the re-sampler 10 suppliesto a second of the two input ports of the time division multiplexer 5.The re-sampler 10 halves the code rate of its response by inserting arespective X₁ bit received from an X₁ bits generator 11 after each ofthe X₂ bits it receives from the lateral (207, 187) R-S FEC encoder 8.

A convolutional interleaver 12 is connected for receiving the successivedata segments of the non-interleaved data field assembled by thetime-division multiplexer 5. The convolutional interleaver 12 respondsto supply the successive data segments of an interleaved data fieldusing interleaving as prescribed by A/53, Annex D, §§ 4.2.4 titled“Interleaving”. A precoder 13 is connected for receiving the X₂ bits ofthe convolutional interleaver 12 response and generating Z₂ bits byadding modulo-2 the X₂ bits with those bits from twelve symbol epochsprevious. A 12-phase trellis encoder 14 is connected for receiving theX₁ bits of the convolutional interleaver 12 response and supplying themas Z₁ bits. The trellis encoder 14 is connected for supplying Z₀ bitsthat it generates dependent on previously received X₁ bits. A symbol mapread-only memory 15 is connected for receiving Z₂ bits from the precoder13 as a portion of its addressing input signal and for receiving the Z₁and Z₀ bits form the trellis encoder 14 as the remaining portion of itsaddressing input signal. The trellis encoder 14, the precoder 13 and thesymbol map ROM 15 conform with the 8VSB trellis encoder precoder andsymbol mapper shown in FIG. 7 of A/53, annex D. The precoder 13 thetrellis encoder 14 and the symbol map ROM 15 are operated in conformancewith A/53, Annex D, §§ 4.2.5 titled “Trellis coding”.

The symbol map ROM 15 operates as a symbol mapper supplying 3-bit,8-level symbols to a first-in/first-out buffer memory 16. The FIFObuffer memory 16 is operated to provide rate buffering and to open upintervals between 828-symbol groups in the symbol stream supplied to asymbol-code assembler 17, into which intervals the symbol-code assembler17 inserts synchronizing signal symbols. Each of the successive datafields begins with a respective interval into which the symbol-codeassembler 17 inserts symbol code descriptive of adata-segment-synchronization (DSS) sequence followed by a symbol codedescriptive of an initial data segment including an appropriatedata-field-synchronization (DFS) sequence. Each data segment in therespective remainder of each data field is followed by a respectiveinterval into which the symbol-code assembler 17 inserts symbol codedescriptive of a respective DSS sequence. Apparatus 18 for inserting theoffset to cause pilot is connected to receive assembled data fields fromthe symbol-code assembler 17. The apparatus 18 is simply a clockeddigital adder that zero extends the number used as symbol code and addsa constant term thereto to generate a real-only modulating signal indigital form, supplied to a vestigial-sideband amplitude-modulationdigital television transmitter 19 of conventional construction.

FIG. 2 shows one particular construction 110 of the X₁ bits generator 11for the FIG. 1 DTV transmitter. A read only memory 111 responds to inputaddressing received from a symbol counter 112 to supply X₁ bits to there-sampler 10 in the FIG. 1 DTV transmitter. FIG. 3 is a table showing apossible set of X₁ bits stored in the ROM 111. The symbol counter 112 isof a type supplying consecutive counts zero through forty-seven androlling back to zero count after forty-seven count. If the trellsencoder 14 receives X₁ bits that change value every second X₁ bit foreach of the twelve trellis coding phases, the trellis encoder 14generates all four types of Z₁, Z₀ pairs substantially the same numberover a long enough period of time. By staggering the way the X₁ repeatsoccur in the twelve trellis coding phases, the length of this period oftime can be shortened some.

However, there is a preference that each grouping of thehalved-code-rate signal in the convolutional interleaver 12 responsecontains 48 or a multiple of 48 successive symbols. This can be achievedmost of the time by grouping the halved-code-rate signal in thetime-division multiplexer 5 response so it occurs in bands of twelvecontiguous data segments.

Since the X₂ bits are randomized the Z₂ bits supplied from the precoder13 are also randomized. The randomized nature of the Z₂ bits, all fourtypes of Z₁, Z₀ pairs occurring in substantially the same number over aperiod of time, and the independence of the Z₂ and Z₁ bits cause theeight 8VSB symbols to occur substantially as often as each other in therobust modulation. Accordingly, the ratio of peak power to average powerin the robust modulation generated in response to the particularconstruction 110 of the X₁ bits generator 11 is substantially the sameas in normal 8VSB modulation.

FIG. 2 shows a detector 113 of the start of the data field connected tosupply the symbol counter 112 with a reset pulse at the beginning ofeach data field, which reset pulse resets the count to 0000000. Atypical construction for the detector 113 includes a match filter forgenerating a pulse response to the PN511 sequence in the initial datasegment of the data field DFS. The typical construction for the detector113 further includes a clocked digital delay line for delaying thatpulse response to provide the reset pulse to the symbol counter 112 toreset it to the 0000000 court at the beginning of the actual data field,exclusive of synchronizing signals.

FIG. 4 shows a modification that can be made to the FIG. 1 DTVtransmitter that provides for the transmission of 8VSB and pseudo-2VSBsignals in time-division multiplex. The re-sampler 10 and the X₁ bitsgenerator 11 of the FIG. 1 DTV transmitter are replaced by a re-sampler20. The re-sampler 20 halves code rate in the data stream it supplies tothe time-division multiplexer 5 used to assemble data fields. There-sampler 20 halves code rate by immediately repeating each X₂ bit,thereby generating a respective X₁ bit. The pre-coder 13, the trellisencoder 14 and the symbol map ROM 15 convert each of the resulting bitpairs to a respective pseudo-2VSB symbols.

FIG. 5 shows a modification of the FIG. 1 DTV transmitter that providesfor the transmission of enhanced-4VSB signal in time-division multiplexwith normal 8VSB signal. Circuitry 21 to generate the Y₁ bits for E-4VSBis interposed between the convolutional interleaver 12 and the trellisencoder 14. A selector 211 of the source of Y₁ bits is connected forsupplying Y₁ bits to the trellis encoder 14. When a normal 8VSB symbolsis to be transmitted, the selector 211 generates the Y₁ bit byreproducing the X₁ bit from the convolutional interleaver 2 response.Whenever an E-4VSB symbol is to be transmitted, the selector 211generates the Y₁ bit by reproducing the response from an exclusive-NORgate 212. The exclusive-NOR gate 212 is connected for receiving eachsuccessive Y₂ bit from the precoder 13 at one of its two input ports andfor receiving each successive Z₀ bit from the trellis encoder 4 at theother of its two input ports. The Y₂ bit from the precoder 13corresponds to the Z₂bit from the trellis encoder 14, so effectively theZ₁ bit of each E-4VSB symbol that is to be transmitted is theexclusive-NOR gate 212 response to its Z₂ and Z₀ bits. If the Z₂ and Z₀bits of the E-4VSB symbol are ZERO and ONE, respectively the E-4VSBsymbol must have a −5 symbol code with a Z₁ bit, that is a ZERO. If theZ₂ and Z₀ bits of the E-4VSB symbol are both ZEROes, the E-4VSB symbolmust have a −3 symbol code with a Z₁ bit that is a ONE. If the Z₂ and Z₀bits of the E-4VSB symbol are ONE and ZERO, respectively, the E-4VSBsymbol must have a +1 symbol code with a Z₁ bit that is a ZERO. If theZ₂ and Z₀ bits of the E-4VSB symbol are both ONEs, the E-4VSB symbolmust have a +7 symbol code with a Z₁ bit that is a ONE.

FIG. 5 shows a convolutional bit interleaver 22 connected for supplyingthe control signal for the selector 211. The convolutional bitinterleaver 22 is connected for receiving bits that map the position ofbytes in the a non-interleaved data field from a generator 23 of such abyte map. Bytes in data segments that are to be transmitted withordinary 8VSB symbols are coded with one of the bit values ZERO and ONE.Bytes in data segments that are to be transmitted with E-4VSB symbolsare coded with the other of the bit values ZERO and ONE. In its responsethe convolutional bit interleaver 22 interleaves the bits the generator23 supplies analogously to the way that the convolutional interleaver 12interleaves bytes of the non-interleaved data field in its response.Accordingly, the convolutional bit interleaver 22 generates bits ofcontrol signal for the selector 211 that map the position of bytes inthe interleaved data field supplied from the convolutional interleaver12. The bits of this control signal for the selector 211 indicatewhether the symbols extracted from that byte are to be ordinary 8VSBsymbols or are to be E-4VSB symbols instead.

FIG. 6 shows a modification of FIG. 1 DTV transmitter in which datapackets in the ancillary transport stream are provided transverseReed-Solomon forward-error-correction coding. A time-divisionmultiplexer 25 has a first input port connected for receiving 207-bytedata segments from the lateral (207, 187) R-S FEC encoder 9 of a firsttype. The time-division multiplexer 25 has a second input port connectedfor receiving 207-byte data segments from a lateral (207, 187) R-S FECencoder 26 of a second type. The time-division multiplexer 25 has anoutput port at which 207-byte data segments from the R-S FEC encoders 9and 26 are reproduced, connected for supplying these data segments to arandom-access memory 27 for being written to one of two banks therein.The RAM 27 stores one 8-bit byte of code plus any byte extensions ateach of its addressed storage locations. The RAM 27 has enough addressedstorage locations to store at least two successive supergroups of (H+K)207-byte data segments apiece.

After H successive ancillary-service data packets have been written intoa bank of the RAM 27, read addressing is applied to this bank. This readaddressing scans these H successive data segments in transversedirection to read H-byte transverse data segments to a transverse (G, H)Reed-Solomon forward-error-correction encoder 28. A data assembler 29assembles the parity bytes generated byte transverse R-S FEC encoder 28to K 187-byte packets with no headers. The data assembler 29 supplieseach of these K packets to the lateral (207, 187) R-S FEC encoder 26 ofsecond type to generate a respective one of K 207-byte data segments.The time-division multiplexer 25 reproduces these K data segments forbeing written into a bank of the RAM 27 to complete the supergroup thatis temporarily stored therein. The (H+K) data segments in this completedsupergroup are then read seriatim from that bank of the RAM 27 to there-sampler 10 at appropriate intervals.

Preferably, the K data segments containing parity bytes from transverseR-S FEC coding are read from the RAM 27 before the H data segmentscontaining the payload data selected for robust transmission. Thisprocedure enables (or helps) a DTV receiver of new design to determinewhen the earliest of a supergroup of (H+K) data segments is received.This is important because the supergroups of (H+K) data segments areformed from selected ones of successive data segments, which selecteddata segments are scattered through one or more data fields. Thesupergroups of (H+K) data segments need not have defined boundariesrespective to data fields as defined in A/53. A DTV receiver of newdesign can determine that lateral (207, 187) R-S FEC coding of secondtype is used in each of the K segments that contain parity bytes fromtransverse R-S FEC coding with correctable byte errors. A DTV receiverof new design can also determine the type of lateral (207, 187) R-S FECcoding used in ones of these K segments that contain parity bytes fromtransverse R-S FEC coding with no greater a number of byte errors thantwice the number of correctable byte errors. A determination thatlateral (207, 187) R-S FEC coding of second type is used in a datasegment conditions the DTV receiver to temporarily store the datasegment in a bank of memory for a supergroup of (H+K) data segments. TheDTV receiver is further conditioned to select subsequent data segmentsof the same supergroup also to be temporarily stored in that bank ofmemory. The DTV receiver then proceeds to perform transverse R-S FECdecoding of the supergroup of (H+K ) data segments. The type of lateral(207, 187) R-S FEC coding used in the K-data segments containing paritybytes from transverse R-S FEC coding can specify the type of H datasegments that should be selected for temporary storage in the supergroupof (H+K) data segments. These H data segments are identified by the PIDsin their headers and the continuity counts in the headers can be usedfor determining when the supergroup of (H+K) data segments temporarilystored in a bank of memory is completed.

The R-S FEC coding used by the lateral (207, 187) R-S FEC encoder 26 ofsecond type is shortened differently than the R-S FEC coding used by thelateral (207, 187) R-S FEC encoder 9 of first type. The first type of(207, 187) R-S FEC coding is that implicitly specified in A/53 and ispresumably shortened from a (255, 235) R-S FEC code using forty-eightvirtual bytes that are all 0000 0000. Other types of (207, 187) R-S FECcoding can be generated by modifying at least ten selected bytes of itsR-S FEC coding in a prescribed way, such as one's complementing each bitin the selected bytes. Alternatively, other types of (207, 187) R-S FECcoding can be generated using different sets of virtual bytes that arenot all 0000 0000. Such modifications of R-S FEC coding are described inmore detail U.S. patent application Ser. No. 10/885,460 filed 6 Jul.2004 for Allen LeRoy Limberg and titled “Reed-Solomon CodingModifications for Signaling Transmission of Different Types of DataPackets”.

The FIG. 6 DTV transmitter can be modified to provide for thetransmission of 8VSB and pseudo-2VSB signals time-division multiplex.The lateral (207, 187) R-S FEC encoder 26 of second type is replaced bya lateral (207, 187) R-S FEC encoder 26 of third type, which identifiesthose data segments used for pseudo-2VSB transmission. The re-sampler 10and the X₁ bits generator 11 of the FIG. 6 DTV transmitter are replacedby the re-sampler 20 of FIG. 4. The re-sampler 20 halves code rate inthe data stream it supplies to the time-division multiplier 5 used toassemble data fields.

FIG. 7 shows another modification that can be made to the FIG. 1 DTVtransmitter, which modification provides transverse R-S FEC coding todata transmitted using the full alphabet of 8VSB symbols. Atime-division multiplexer 31 has a first input port connected forreceiving 207-byte data segments from the lateral (207, 187) R-S FECencoder 4 of first type. The time-division multiplexer 31 has a secondinput port connected for receiving 207-byte data segments from a lateral(207, 187) R-S FEC encoder 32 of a fourth type. The time-divisionmultiplexer 31 has an output port at which 207-byte data segments fromthe R-S FEC encoders 4 and 32 are reproduced. This output port isconnected for supplying these data segments to a random-access memory 33for being written to one of two banks therein. The RAM 33 stores one8-bit byte of code plus any byte extensions at each of its addressedstorage locations. The RAM 33 has enough addressed storage locations tostore at least two successive supergroups of (N+Q) 207-byte datasegments apiece. (N+Q) is presumed to be 156 or a multiple thereof,which simplifies keeping track of the phasing of data randomization inthe DTV transmitter and in DTV receivers.

After N successive data segments have been written into a bank of theRAM 33, read addressing is applied this bank. This read addressing scansthese N successive data segments in transverse direction to read H-bytetransverse data segments to a transverse (M, N) Reed-Solomonforward-error-correction encoder 34. A data assembler 35 assembles theparity bytes generated by the reverse R-S FEC encoder 34 into Q 187-bytedata packets with no headers. The data assembler 35 supplies each ofthese Q packets to the lateral (207, 187) R-S FEC encoder 32 of fourthtype to generate a respective one of Q 207-byte data segments. Thetime-division multiplexer 31 reproduces these Q data segments for beingwritten into a bank of the RAM 33 to complete the supergroup that istemporarily stored therein. The (N+Q) 207-byte data segments in thiscompleted supergroup are then read seriatim from that bank of the RAM 33to the first input port of the time-division multiplexer 5 atappropriate intervals. The second input port of the time divisionmultiplexer 5 is connected to receive 207-byte data segments from there-sampler 10.

Generally, it is preferable that the Q data segments containing paritybytes from transverse R-S FEC coding are read from the RAM 33 after theN data segments containing the payload data selected for transverse R-SFEC coding. In many instances the transversal R-S FEC coding oversupergroups of (N+Q) data segments involves more transverse paths thanthere are bytes in a packet assembled by the data assembler 35, so thereis a progressive skew in the transverse paths as they traverse thecorrection field. If transversal R-S FEC coding is done on the paritybytes of the lateral (207, 187) R-S FEC coding of data segments in theinformation field, for example, there will be 207 transverse paths. Eachsuccessive set of 207 parity bytes will occupy more than the 187 bytesavailable in each data packet assembled by the data assembler 35, and sowill have to be assembled within two consecutive data packets. Thedistance between bytes in the same transverse path is lengthened whencrossing from the information field into the correction field if the Qdata segments containing transverse R-S FEC coding are read from the RAM33 after the N data segments containing the payload data. If the Q datasegments containing transverse R-S FEC coding are read from the RAM 33before the N data segments containing the payload data, the distancebetween bytes in the same transverse path is shortened when crossingfrom the information field into the correction field. This impairs thecapability to withstand certain burst errors. Since all data segmentsexcept those containing DFS are contained in successive (N+Q)supergroups, a DTV receiver of new design temporarily stores all datasegments in memory for possible transverse R-S FEC decoding. This isautomatic. The DTV receiver of new design does not need to be promptedto this action responsive to information identifying the type oftransverse R-S FEC coding included in the Q segments containing paritybytes from transverse R-S FEC code. So, there is no need to positionthese Q segments at the beginning of the supergroup.

The FIG. 7 DTV transmitter modified to provide for the transmission of8VSB and pseudo-2VSB signals in time-division-multiplex. In the modifiedFIG. 7 DTV transmitter the re-sampler 20 of FIG. 4 replaces there-sampler 10 and the X₁ bits generator 11.

FIG. 8 shows a further modification of the FIG. 1 DTV-transmitter, whichmodification provides transverse R-S FEC coding to data subsequentlytransmitted using a restricted alphabet of 8VSB symbols as well as todata subsequently transmitted using the full alphabet of 8VSB symbols.The program source 1 of a principal transport stream is connected forwriting data packet to the FIFO buffer memory 2 for temporary storagetherein. The data randomizer 3 is connected for receiving data packetsread from the FIFO buffer memory 2 and randomizing the bits in thosedata packets. The program source 6 of an ancillary transport stream isconnected for writing data packets to the FIFO buffer memory 7 fortemporary storage therein. The data randomizer 8 is connected forreceiving data packets read from the FIFO buffer memory 7 andrandomizing the bits in those data packets. A first input port of atime-division multiplexer 36 is connected to receive randomized datapackets from the data randomizer 3 and the second input port of themultiplexer 36 is connected to receive randomized data packets from thedata randomizer 8. The multiplexer 36 reproduces these 187-byterandomized data packets in a time-division multiplexed response suppliedfrom the output port of the multiplexer 36 to the input port of alateral (207, 187) R-S FEC encoder 37 of the first type. The lateral(207, 187) R-S FEC encoder 37 converts these randomized 187-byte datapackets to respective 207-byte segments of lateral (207, 187)Reed-Solomon forward-error-correction coding that appends the respectivetwenty parity bytes of the coding of each randomized 187-byte datapacket after the conclusion thereof. This complies with the practicespecified in A/53, Annex D, §§ 4.2.3 titled “Reed-Solomon encoder.

A first input port of a time-division multiplexer 38 is connected toreceive the 207-byte segments of lateral (207, 187) R-S FEC codinggenerated by the lateral (207, 187) R-S FEC encoder 37. A second inputport of the time-division multiplexer 38 is connected to receive207-byte segments of nulls generated by a null segment generator 39. Thenull segment generator 39 continuously generates 207-byte segments ofnull bytes. The time-division multiplexer 38 is operated so that one ofthese segments of null bytes is reproduced in its response immediatelybefore each 207-byte segment supplied from the lateral (207, 187) R-SFEC encoder 37 is reproduced. A third input port of the time-divisionmultiplexer 38 is connected to receive 207-byte segments of lateral(207, 187) R-S FEC coding generated by a lateral (207, 187) R-S FECencoder 40 of fifth type.

A random-access memory 41 is connected to an output port of thetime-division multiplexer 38, which supplies 207-byte data segments forbeing written to one of two banks of memory in the RAM 41. The RAM 41stores one 8-bit byte of code-plus any byte extensions at each of itsaddressed storage locations. The RAM 41 has enough addressed storagelocations to store at least two successive supergroups of (N+Q) 207-bytedata segments apiece.

After N successive data segments have been written into a bank of theRAM 41, read addressing is applied to this bank. This read addressingscans these N successive data segments in transverse direction to readH-byte transverse data segments to a transverse (M, N) Reed-Solomonforward-error-correction encoder 42. A data assembler 43 assembles theparity bytes generated by the transverse R-S FEC encoder 42 into Q187-byte data packets with no headers. The data assembler 43 supplieseach of these Q packets to the lateral (207, 187) R-S FEC encoder 40 offourth type to generate a respective one of Q 207-byte data segments.The time-division multiplexer 38 reproduces these Q data segments forbeing written into a bank of the RAM 41 to complete the supergroup thatis temporarily stored therein.

After transverse R-S FEC coding is completed, the (N+Q) data segments ineach completed supergroup are read in prescribed order from the RAM 34to the re-sampler 10, as well as to the first input port of thetime-division multiplexer 5. This prescribed order of reading isgenerally serial in character, but reverses the order in which a nulldata segment and the immediately succeeding data segment in thesupergroup are read from the RAM 34 as a pair of successive datasegments. The immediately succeeding data segment is read from the RAM34 one data segment interval early, so the pair of data segmentsgenerated by the re-sampler 10 is timed so as to be able to replace thepair of successive data segments read from the RAM 34. The time-divisionmultiplexer 5 assembles data fields by time-division multiplexing pairsof data segments received from the re-sampler 10 with selected ones ofthe data segments read from the RAM 34.

The FIG. 8 DTV transmitter can be modified to provide for thetransmission of 8VSB and pseudo-2VSB signals in time-division multiplex.In the modified FIG. 8 DTV transmitter the re-sampler 20 of FIG. 4replaces the re-sampler 10 and the X₁ bits generator 11.

FIGS. 9A, 9B and 9C combine to form a FIG. 9 schematic diagram of a DTVreceiver capable of receiving DTV signals as transmitted by the DTVtransmitters described supra. The FIG. 9A portion of the DTV receiverincludes a vestigial-sideband amplitude-modulation (VSB AM) DTV receiverfront-end 44 for selecting a radio-frequency DTV signal for receptionconverting the selected RF DTV signal to an intermediate-frequency DTVsignal, and for amplifying the IF DTV signal. An analog-to-digitalconverter 45 is connected for digitizing the amplified IF DTV signalsupplied from the DTV receiver front-end 44. A demodulator 46 isconnected for demodulating the digitized VSB AM IF DTV signal togenerate a digitized baseband DTV signal, which is supplied to digitalfiltering 47 for equalization of channel response and for rejection ofco-channel interfering NTSC signal. Synchronization signals extractioncircuitry 48 is connected for receiving the digital filtering 47response. Responsive to data-field-synchronization (DFS) signals, thesync signals extraction circuitry 48 detects the beginnings of dataframes and fields. Responsive to data-segment-synchronization (DSS)signals, the sync signals extraction circuitry 48 detects the beginningsof data segments.

FIG. 9A shows circuitry for analyzing the symbol alphabet used invarious portions of the reproduced baseband DTV signal. This circuitryincludes a hard-decision decoder 49 for 8VSB symbols, which is connectedfor receiving the response of the digital filtering 47 for equalizationof channel response and for rejection of co-channel interfering NTSCsignal. The decisions that the decoder 49 makes concerning the 3-bitsymbols are supplied to a de-interleaver 50 that complements theconvolutional interleaver 12 in the DTV-transmitter. However thede-interleaver 50 operates with 12-bit bytes, rather than standard 8-bitbytes, and supplies symbol code to circuitry 51 to decide the symbolalphabet used in each data segment. The circuitry 51 decides the symbolalphabet used in each data segment by evaluating the distribution of8VSB symbols used in each data segment, which procedures are describedin more detail further on in this specification. Assuming that besidesthe full 8VSB alphabet two or three restricted alphabets are used, thedecisions that the circuitry 51 supplies are expressed as bit pairs.E.g., 00 indicates full 8VSB alphabet; 01 indicates pseudo-2VSB; 10indicates E-4VSB; 11 indicates a restricted alphabet that selectsbetween two groups of possible symbols. The first group of possiblesymbols consists of symbols with −7, −5, +1 and +3 normalized modulationlevels. The second group of possible symbols consists of symbols with−3, −1, +5 and +7 normalized modulation levels.

The circuitry 51 can determine in the following way whether or not adata segment is transmitted using pseudo-2VSB. The de-interleaver 50supplies the circuitry 51 with a succession of 3-bit symbol codes. TheZ₂ and Z₁ bits of these symbol codes are applied to respective inputports of a first two-input exclusive-NOR gate, which responds with a ONEto all symbols included in the pseudo-2VSB set and with a ZERO to allsymbols excluded from the pseudo-2VSB set. The ONEs that the firstexclusive-NOR gate generates in the 828 symbol epochs of each datasegment are counted. The count is compared to a prescribed thresholdvalue, such as 777. If this threshold is exceeded, this is an indicationthat the data segment was transmitted using pseudo-2VSB. This indicationconditions a first pair of tri-states to assert the 01 bit pair from lowsource impedances on the output lines from the circuitry 51.

The circuitry 51 can determine in the following way whether or not adata segment is transmitted using E-4VSB of the sort proposed byETRI/CNU. The 3-bit symbol codes that the de-interleaver 50 supplies aresupplied to a set of eight decoders, each responding with a ONE when andonly when a respective one of the eight 3-bit symbol codes occurs. Theresponses of the decoders for 001, 010, 100 and 111 symbol codes areapplied to respective input ports of a 4-input OR gate. The ONEs thatthe 4-input OR gate generates in the 828 symbol epochs of each datasegment are counted. The count is compared to a prescribed thresholdvalue, such as 777. If this threshold is exceeded, this is an indicationthat the data segment was transmitted using E-4VSB of the sort proposedby ETRI/CNU. This indication conditions a second pair of tri-states toassert the 10 bit pair from low source impedances on the output linesfrom the circuitry 51.

The circuitry 51 can determine in the following way whether or not adata segment is transmitted using symbols with a predetermined sequenceof Z₁ bits. The Z₁ bits of the 3-bit symbol codes that thede-interleaver 50 supplies are applied to a first input port of a secondtwo-input exclusive-NOR gate, which has the prescribed sequence of Z₁bits applied to its second input port. The ONEs that the secondexclusive-NOR gate generates in the 828 symbol epochs of each datasegment are counted. The count is compared to a prescribed thresholdvalue, such as 777. If this threshold is exceeded, this is an indicationthat the data segment was transmitted using symbols with a predeterminedsequence of Z₁ bits. This indication conditions a third pair oftri-states to assert the 11 bit pair from low source impedances on theoutput lines from the circuitry 51.

The circuitry 51 can determine in the flowing way that a data segment istransmitted using the full alphabet of 8VSB symbols. Respective counterscan be used to count the ONES in each of the response of the set ofeight decoders, each responding with a ONE when and only when arespective one of the eight 3-bit symbol codes occurs. The counts can becompared to a threshold value somewhat above 104, say 27, to determineif one of the symbol codes appears more frequently than would beexpected in an 8VSB signal. A plural-input NOR gate is connected forreceiving these eight decisions and the decisions concerning whether ornot the data segment was transmitted using pseudo-2VSB, E-4VSB asproposed by ETRI/CNU, or symbols with a predetermined sequence of Z₁bits. The response of this plural-input NOR gate being a ONE at theconclusion of a data segment is a reasonably reliable indication thatthe data segment was transmitted using the full alphabet of 8VSBsymbols.

The bit pairs coding the circuitry 51 decisions are supplied to a mapper52 of the byte pattern in the de-interleaved data field. The mapper 52extends each bit pair decision by repeating it 206 times, to map the 207bytes of a data segment as a line of bit pair decisions. A convolutionalinterleaver 53 generates the pattern of bit pair decisions mapping bytecharacteristics in the interleaved data field of the baseband DTV signalsupplied as response from the digital filtering 47 for equalization ofchannel response and for rejection of co-channel interfering NTSCsignal.

Digital delay circuitry 54 delays the digital filtering 47 response by105 or so data segments to temporally align it with the bit pairs fromthe convolutional interleaver 53 that describe symbol use in theinterleaved data field. A plural-mode 12-phase trellis decoder 55 ofViterbi type is connected for receiving the digital filtering 47response as delayed by the digital delay circuitry 54. When the bit pairdecisions from the convolutional interleaver 53 indicaterestricted-alphabet symbols are currently being supplied to theplural-mode trellis decoder 55, the decision tree in the trellisdecoding is selectively pruned. This pruning excludes decisions thatcurrently received symbols have normalized modulation levels that areexcluded from the restricted alphabet of 8VSB symbols currently in use.The trellis decoder 55 is connected to supply bytes of data to ade-interleaver 56 that complements the convolutional interleaver 12 inthe DTV transmitter.

More particularly, circuitry similar to that shown in FIG. 2 isassociated with the plural-mode 12-phase trellis decoder 55 of Viterbitype. This circuitry provides the trellis decoder 55 informationconcerning which symbols are precluded at which locations in the datafield when the convolutional interleaver 53 supplies the trellis decoder55 the bit pair 11 as a control signal. The bit pair 11 indicates thatthe symbols trellis decoder 55 is receiving are from a restrictedalphabet that selects between two groups of possible symbols. Symbolstransmitted at −3, −1, +5 and −7 a normalized modulation levels areprecluded from locations in the data field reserved for the first groupof possible symbols. Symbols transmitted at −7, −5, +1 and +3 normalizedmodulation levels are precluded from locations in the data fieldreserved for the second group of possible symbols. The ranges ofdecision in the plural-mode 12-phase trellis decoder 55 are adjusted toaccommodate the decision tree being pruned in a time-dependent way aslocations in the data field are scanned.

When the convolutional interleaver 53 supplies the bit pair 00 as acontrol signal indicating to the plural-mode 12-phase trellis decoder 55that the symbols it currently receives are from ordinary 8VSBtransmission, the ranges of decision in the trellis decoder 55 are theconventional ones for receiving A/53 DTV broadcasts. The decision treein the plural-mode 12-phase trellis decoder 55 is not pruned. When theconvolutional interleaver 53 supplies the bit pair 01 as a controlsignals indicating to the trellis decoder 55 that the symbols itcurrently receives are from pseudo-2VSB transmission, the ranges ofdecision are adjusted and the decision tree is pruned in the trellisdecoder 55. This is done in such way as to preclude −3, −1, +1 and +3symbol decisions. When the convolutional interleaver 53 supplies the bitpair 10 as a control signal indicating to the trellis decoder 55 thatthe symbols it currently receives are from E-4VSB transmission, theranges of decision are adjusted and the decision tree is pruned in thetrellis decoder 55. This is done in such way that so as to preclude, −7,−1, +3 and +5 symbol decisions.

Information concerning the symbol sets used for generating each datasegment in the de-interleaved data field can be encoded in the“reserved” portions of the data field synchronization data segments, asknown in the prior art. Such information can be decoded and used tovalidate circuitry 51 response. Alternatively, such information can beused by the mapper 52 instead of the circuitry 51 response fordetermining the pattern of data segments in the de-interleaved datafield that are transmitted using symbols from a restricted alphabet.This avoids the need for the digital delay 54. This facilitateshard-decision decoding on which adaptation of the equalization and NTSCrejection filtering is based being constructed to depend on the bit-pairdecisions that the convolutional interleaver 53 supplies as to thenature of received symbols, so that tracking of dynamic multipath can beimproved.

A novel feature of the FIG. 9 DTV receiver is a 2-segments-to-1 datacompressor 57 for data segments decoded from restricted-alphabetsymbols. The data compressor 57 is connected for receiving from thede-interleaver 56 successive data segments of de-interleaved datafields. The data compressor 57 is connected for receiving from digitaldelay circuitry 58 bit pairs indicating previous decisions made by thecircuitry 51 concerning whether the data segments the de-interleaver 56currently supplies were or were not decoded from 8VS-B symbols that hadalphabet restrictions. The digital delay circuitry 58 delays these bitpairs 104 data segments plus the latent delay of the trellis decoder 55.Supposing a 00 bit pair indicates full 8VSB alphabet, the bits in thebit pair from the circuitry 51 can be ORed to generate indications ofwhether data were or were not decoded from 8VSB symbols that hadalphabet restrictions. The digital delay circuitry 58 can then bemodified to delay these single-bit indications rather than bit-pairindications.

The data compressor 57 is selective in operation, its responsereproducing without modification data segments decoded from 8VSB symbolsthat had no alphabet restrictions. The data compressor 57 converts eachpair of data segments decoded from restricted-alphabet symbols to arespective single data segment. The data compressor 57 treats the pairof data segments as a succession of X₂, X₁ bit pairs and eliminates theX₁ bits to leave a succession of X₂ bits. This succession of X₂ bitsreproduces the single data segment at original code rate that the DTVtransmitter used to generate the pair of data segments at halved coderate.

The trellis decoder 55 can be designed to supply an extension to eachbyte it supplies, which extension comprises one or more additional bitsindicative of the confidence level that the byte is correct. Thede-interleaver 56 and the 2-segments-to-1 data compressor 57 can bedesigned to preserve those byte extensions in their responses, so thosebyte extensions are available to help locate byte errors in subsequentR-S FEC decoding procedures. The 2-segments-to-1 data compressor 57 isconnected for supplying its response to lateral (207, 187) R-S FECdecoding apparatus 59 shown in FIG. 9B.

FIGS. 9B and 9C show parts 60(A) and 60(B), respectively, of operationscontrol circuitry 60 for controlling transverse Reed-Solomonforward-error-correction decoding procedures. Showing the operationscontrol circuitry 60 in two parts is an artifice used in the drawings toavoid running numerous connections from elements shown in FIGS. 9A and9B to elements shown in FIG. 9C. FIG. 9B shows the operations controlcircuitry 60 connected for receiving DFS signal, DSS signal and clockingsignal at an even multiple of symbol rate via respective connectionsfrom the sync signals extraction circuitry 48 in FIG. 9A. These signalsare provided with respective delays by means not explicitly shown, whichdelays compensate for latent delays accumulated in the FIG. 9A circuitryand in the lateral (207, 187) R-S FEC decoding apparatus 59 shown inFIG. 9B. FIG. 9B shows the operations control circuitry 60 connected forreceiving the response of the digital delay circuitry 58 in FIG. 9A,which response provides indications of whether data segments were orwere not decoded from 8VSB symbols that had alphabet restrictions.

A de-randomizer 61 is connected for providing de-randomized response to187-byte data packet portions of corrected data segments supplied fromthe lateral (207, 187) R-S FEC decoding apparatus 59. Header detectionapparatus 62 detects the PID portions of the de-randomized data packetsto provide the operations control circuitry 60 information concerningthe types of corrected data segments supplied from the lateral (207,187) R-S FEC decoding apparatus 59. The operations control circuitry 60uses this information when transverse R-S FEC decoding is to beperformed only on selected types of data segments. A bankedrandom-access memory 63 is employed in certain transverse R-S FECdecoding procedures. Writing to and reading from the banks of the RAM 63is controlled by the operations control circuitry 60.

The lateral (207, 187) R-S FEC decoding apparatus 59 is connected forsupplying successive bytes of corrected data segments to the RAM 63 tobe written into one of two banks of memory therein. Each of these banksof memory is capable of storing the (N+Q) data segments in a supergroup.Each addressed location in the RAM 63 is capable of temporarily storinga byte supplied from the lateral (207, 187) R-S FEC decoding apparatus59, plus any extension or extensions of that byte. Consider successivesupergroups of (N+Q) data segments to be ordinally numbered. Therespective cycles of operation for the two banks of the RAM 63 areshifted with respect to each other in time. This shift is such thatbytes of odd-numbered supergroups of (N+Q) data segments are written toone bank, and bytes of even-numbered supergroups of (N+Q) data segmentsare written to the other bank. The RAM 63 is operated so that, whilebytes of a newly received supergroup of (N+Q) data segments are beingwritten to one bank of the memory, the previous supergroup of (N+Q) datasegments that was written to the other bank of memory can be correctedfor byte errors. Writing each successive byte of a newly receivedsupergroup of (N+Q) data segments to an addressed storage location inone bank of the RAM 63 is preceded by reading from that storage locationa byte from two such supergroups previous. If (N+Q) equals 156 or amultiple thereof, a data segment read from the RAM 63 will occupy thesame position in a data field that it had when written into the RAM 63,which simplifies subsequent data de-randomization of data packets.

The operations control circuit 60 supplies the addressing for writingand reading operations of the RAM 63. The operations control circuitry60 includes counter circuitry for counting at an even multiple of therate-bytes are supplied from the lateral (207, 187) R-S FEC decodingapparatus 59. The count from this counter circuitry is synchronized withthe received data fields and data segments using the synchronizingsignals extracted by the synchronization signal extraction circuitry 48.Portions of the count from this counter provides read addressing to apair of read-only memories. These ROMs respectively generate theaddressing supplied to each bank of the RAM 63. Storage locations in oneof the RAM 63 banks are addressed by row and by column for being read toa lateral (207, 187) Reed-Solomon forward-error-correction decodingapparatus 64 and then overwritten with a supergroup of (N+Q) datasegments supplied from the lateral (207, 187) R-S FEC decoding apparatus59. Successive addresses occur at the rate bytes are supplied from thelateral (207, 187) R-S FEC decoding apparatus 59.

The initial writing of a supergroup of (N+Q) data segments into a bankof the RAM 63 has to take into account the effects of data compressionby the 2-segments-to-1 data compressor 57. The operations controlcircuitry 60 is connected for receiving the response of digital delaycircuitry 57, which response includes indication of the initial datasegment in a pair of data segments transmitted using a restricted symbolalphabet. The operations control circuitry 60 arranges for the RAM 63 tobe written with a segment of null bytes during the portion of thede-interleaved data field that was originally occupied by the initialdata segment in a pair of data segments transmitted using a restrictedsymbol alphabet. This “shortens” the supergroup of (N+Q) data segmentstemporarily stored in the RAM 63 so as to reproduce the supergroup of(N+Q) data segments resulting from transverse R-S FEC coding at thetransmitter.

While a new supergroup of (N+Q) data segments is being written into onebank of the RAM 63, the storage locations in the other of the RAM 63banks are transversally addressed for reading to a selected one of array65 of transverse Reed-Solomon forward-error-correction decoders. Theselection is made by transverse Reed-Solomon forward-error-correctiondecoder application circuitry 66 responsive to a SELECT A control signalsupplied by the operations control circuitry 60. The operations controlcircuitry 60 determines which transverse R-S FEC decoder, if any toselect from information the lateral (207, 187) R-S FEC decodingapparatus 59 supplies. This information concerns the type of segmentsincluding parity bytes of transverse R-S FEC decoding that the R-S FECdecoding apparatus 59 finds to be correctable. After the bytes in eachtransversal path have had errors therein corrected to the extent thetransverse R-S FEC code permits, these bytes are written back to thesame storage locations in this other of the RAM 63 banks they were readfrom.

Successive addresses in the transverse scanning of storage locations ina bank of the RAM 63 occur at a multiple of twice the rate bytes aresupplied from the lateral (207, 187) R-S FEC decoding apparatus 59. Ifonly one type of transverse R-S FEC coding is employed in eachsupergroup of (N+Q) data segments, successive addresses for transversescanning of storage locations in the RAM 63 can occur at only twice therate bytes are supplied from the lateral (207, 187) R-S FEC decodingapparatus 59. If two types of transverse R-S FEC coding are employed ineach supergroup of (N+Q) data segments, independent transverse scanningof storage locations in the RAM 63 for each type of transverse R-S FECcoding may be desired. Successive addresses for such transverse scanshave to be supplied at four times or more the rate bytes are suppliedfrom the lateral (207, 187) R-S FEC decoding apparatus 59. Alternativedesigns in which transverse scanning of each bank of RAM is clockedindependently of the lateral scanning of the other bank of RAM arepossible. For example, such designs can be implemented using dualporting techniques.

The (207, 187) Reed-Solomon forward-error-correction decoding apparatus64 is connected for receiving 207-byte data segments read from the RAM63 after having been corrected insofar as possible by transverse R-S FECdecoding procedures. The (207, 187) R-S FEC decoding apparatus 64performs lateral Reed-Solomon forward-error-correction on these 207-bytedata segments and toggles the Transport Error Indicator (TEI) bit ineach data packet in those segments in which the decoding apparatus 64finds byte errors that still remain uncorrected. A data de-randomizer 67is connected for receiving the portion of each data segment supplied bythe lateral (207, 187) R-S FEC decoding apparatus 64 other than itstwenty R-S FEC code parity bytes as a 187-byte data packet. The datade-randomizer 67 is connected for supplying de-randomized data packetsto header detection apparatus 69 and to a transport streamde-multiplexer 69.

The transport stream de-multiplexer 69 responds to the header detectionapparatus 69 detecting a selected PIDs in certain types of thede-randomized data packets from the data de-randomizer 67 for sortingthose types of de-randomized data packets to appropriate packetdecoders. For example, video data packets are sorted to an MPEG-2decoder 70. The MPEG-2 decoder 70 responds to the TEI bit in a datapacket indicating that it still contains byte errors by not using thepacket and instituting measures to mask the effects of the packet notbeing used. By way of further example, audio data packets are sorted toan AC-3 decoder 71.

The (207, 187) R-S FEC decoding apparatus 64 supplies corrected 207-bytedata segments to a banked random-access memory 72 shown in FIG. 9C. Eachaddressed location in the RAM 71 is capable of temporarily storing abyte supplied from the lateral (207, 187) R-S FEC decoding apparatus 64,plus any extension or extensions of that byte. Each bank of memory inthe RAM 72 is capable of storing the (H+K) data segments in a supergroupused in an ancillary-service transmission. These (H+K) data segments canoccur during a number of supergroups of (N+Q) data segments.

The operations control circuitry 60 controls the writing and readingoperations of the RAM 72. The lateral (207, 187) R-S FEC decodingapparatus 64 notifies the operations control circuitry 60 when one ofthe K segments containing parity bytes for a supergroup of transverse(G, H) R-S FEC coding occurs in the response of the decoding apparatus64 supplied to the RAM 72. Responsive to such notification, theoperations control circuitry 60 enables the writing of this segment intoa bank of the RAM 72. When one of the H data segments in a supergroup oftransverse (G, H) R-S FEC coding occurs in the response of the lateral(207, 187) R-S FEC decoding apparatus 64, it is de-randomized by thedata de-randomizer 67 for application to the header detection apparatus69. The header detection apparatus 69 notifies the operations controlcircuitry 60 of the occurrence of the de-randomized PID of thisde-randomized data segment. Responsive to such notification, theoperations control circuitry 60 enables the writing of this data segmentinto a bank of the RAM 72. A counter within the operations controlcircuitry 60 keeps track of how many of the (H+K) data segments in thesupergroup of transverse (G, H) R-S FEC coding are temporarily stored ina respective bank of the RAM 72. When a full complement of (H+K) datasegments is temporarily stored in a respective bank of the RAM 72, theoperations control circuitry 60 generates addressing that scanstransverse paths through storage locations in that RAM 72 bank. Thesestorage locations are read to a selected one of an array 73 oftransverse Reed-Solomon forward-error-correction decoders. TransverseReed-Solomon forward-error-correction decoder application circuitry 74makes the selection responsive to a SELECT B control signal supplied bythe operations control circuitry 60. The operations control circuitry 60determines which transverse R-S FEC decoder, if any, to select frominformation the lateral (207, 187) R-S FEC decoding apparatus 64supplies. This information concerns the type of segments includingparity bytes of transverse R-S FEC decoding that the R-S FEC decodingapparatus 64 finds to be correctable. After the bytes in eachtransversal path have had errors therein corrected to the extent thetransverse R-S FEC code permits, these bytes are written back to thesame storage locations in the RAM 72 bank they were read from. Theoperations control circuitry 60 generates addressing for reading themdata segments from the RAM 72 bank to a lateral (207, 187) Reed-Solomonforward-error-correction decoder 75.

The (207, 187) Reed-Solomon forward-error-correction decoder 75 isconnected for receiving 207-byte data segments read from the RAM 72after having been corrected insofar as possible by transverse R-S FECdecoding procedures. The (207, 187) R-S FEC decoder 75 performs lateralReed-Solomon forward-error-correction on these 207-byte data segmentsand toggles the Transport Error Indicator (TEI) bit in each data packetin those segments in which the decoder 75 finds byte errors that stillremain uncorrected. A data de-randomizer 76 is connected for receivingthe portion of each data segment supplied by the lateral (207, 187) R-SFEC decoder 74 other than its twenty R-S FEC code parity bytes as a187-byte data packet. The data de-randomizer 76 is connected forsupplying de-randomized data packets to header detection apparatus 77and a transport stream de-multiplexer 78. The header detection apparatus77 responds to the PIDs in the de-randomized data packets to developcontrol signals for the transport stream de-multiplexer 78. Responsiveto these control signals, the transport stream de-multiplexer 78 sortsthe de-randomized data packets to appropriate packet decoders. FIG. 9Cshows a decoder 79 for the data packets of a first ancillary service anda decoder 80 for the data packets of a second ancillary service, eachbeing connected for receiving selected data packets from the transportstream de-multiplexer 78.

The FIG. 9 DTV receiver can be modified so that RAM 72 is written withdata segments selected directly from the response of the lateral (207,187) R-S FEC decoding apparatus 59, rather than from the response oflateral (207, 187) R-S FEC decoding apparatus 64. This avoids the latentdelay associated with temporarily storing data segments in the RAM 63.However, data segments selected directly from thee response of thelateral (207, 187) R-S FEC decoding apparatus 59 will generally containmore byte errors than data segments selected from the response oflateral (207, 187) R-S FEC decoding apparatus 64.

The transverse R-S FEC coding schemes thus far described in thisspecification array the parity bytes for this coding in data segmentsthat have no headers. This permits the parity bytes to be arrayed infewer data segments, reducing overhead and increasing payload. Arrayingthe parity bytes in data segments that have no headers also facilitatesthe transverse code paths being straight and parallel throughout thedata field, supposing that the parity bytes of lateral R-S FEC codingare not subjected to transverse R-S FEC coding.

Transverse R-S FEC coding the parity bytes of the lateral R-S FEC codingof data segments containing payload, as well as the payload bytes,improves the strength of the two-dimensional R-S FEC coding, however.The assembling of data segments containing parity bytes for transverseR-S FEC coding results in transverse paths that skew in the correctionfield relative to their direction in the information payload field.

Arraying the parity bytes in data segments that have no headers has thedisadvantage that there is no continuity count associated with each suchdata segment. Accordingly, when DTV receiver circuitry finds a datasegment to be incapable of correction, it may be harder to determinewhich specific supergroup that data segment may belong to. Alternativeembodiments of the invention are contemplated in which the parity bytesof transversal R-S FEC coding are arrayed in data segments that haveheaders similar to an MPEG-2-compliant data segment, containing a uniquePID and a continuity count. The unique PID for such data segments shouldcause legacy DTV receivers to disregard such data segments, so lateralR-S coding of these data segments can be dispensed with. This saves someoverhead.

Alternative embodiments of the invention are contemplated in which alldata segments have headers similar to an MPEG-2-compliant data segment,containing a unique PID and a continuity count, and also have lateralR-S FEC coding. With knowledge of the disclosure supra such alternativeembodiments are readily designed by one skilled in the art of DTV systemdesign. Such alternative embodiments are explicitly described in U.S.provisional application Ser. No. 6 60/531,124 filed 19 Dec. 2003.

FIG. 10 lists the steps in a routine that can be carried out inconnection with a DTV transmitter as shown in FIG. 1. This routinevalidates that the operation of legacy receivers will not be disruptedby the insertion of restricted-alphabet data segments into thetime-division multiplex (TDM) signal that defines a data field beforesubsequent convolutional interleaving and trellis coding. A segment slotcounter that counts segment slots from one to 312 in a data field andthen rolls over back to one is used in the routine. The count therefromis reset to a number indicative of the segment slot in the data field itis proposed to fill with the final data segment descriptive of a datapacket of symbol codes selected from a restricted alphabet. The datapacket is randomized with the portion of the PRBS associated with thatsegment slot, thereby modeling the projected operation of the datarandomizer 8. The randomized data packet is then (207, 187) R-S FECcoded, thereby modeling the projected operation of the lateral (207,187) R-S FEC encoder 9. The resulting 207-byte data segment is called a“seed” data segment because it grows into a pair of data segments whensubsequently re-sampled to halve its code rate in accordance with aparticular type of alphabet restrictions, modeling the projectedoperation of the re-sampler 10.

The initial data segment in the pair is subjected to (207, 187) R-S FECdecoding to recover a data packet therefrom, thereby modeling projectedoperation of the lateral (207, 187) R-S FEC decoder in a legacy DTVreceiver. If this data packet has a valid PID and its TEI bit indicatesno uncorrected byte error remaining therein, the transport streamde-multiplexer of a legacy DTV receivers would fail to discard the datapacket. So, insertion of the pair of data segments in the proposedsegment slots of the data field is unacceptable. Accordingly, the FIG.10 routine is begun again after incrementing the count supplied from thesegment slot counter.

However if the data packet recovered from the (207, 187) R-S FECdecoding of the initial data segment of the pair has an invalid PID orits TEI bit indicates uncorrected byte error remaining therein the FIG.10 routine continues. The initial data segment in the pair is subjectedto (207, 187) R-S FEC decoding to recover a data packet therefrom,thereby modeling projected operation of the lateral (207, 187) R-S FECdecoder in a legacy DTV receiver. If this data packet has a valid PIDand its TEI bit indicates no uncorrected byte error remaining therein,the transport stream de-multiplexer of a legacy DTV receivers would failto discard the data packet. So, insertion of the pair of data segmentsin the proposed segment slots of the data field is unacceptable.Accordingly, the FIG. 10 routine is begun again after incrementing thecount from the segment slot counter. However, if the data packetrecovered from the (207, 187) R-S FEC decoding of the initial datasegment of the pair as an invalid PID or its TEI bit indicatesuncorrected byte error remaining therein, insertion of the pair of datasegments in the proposed segment slots of the data field is acceptable.

The FIG. 10 routine will usually be carried out in software. Indeed,although FIG. 1 shows hardware for performing operations to generatemodulating signal for the VSB AM DTV transmitter 19, in many DTVtransmitters constructed in accordance with the invention theseoperations will be implemented in software.

FIG. 14 lists the steps in a routine that can be carried out inconnection with a FIG. 1 DTV transmitter modified per FIG. 4. Thisroutine validates that the operation of legacy receivers will not bedisrupted by the insertion of pseudo-2VSB data segments into thetime-division multiplex (TDM) signal that defines a data field beforesubsequent convolutional interleaving and trellis coding. The steps aresimilar to those listed in the FIG. 10 routine, except that there-sampling steps halve code rate by immediately repeating each bit ofthe seed data segment, modeling the projected operation of there-sampler 24.

A time-division multiplex (TDM) signal defines a data field beforesubsequent convolutional interleaving and trellis coding. A much moreelaborate routine than those of FIGS. 10 and 11 is required forvalidating that the operation of legacy receivers will not be disruptedby the insertion of E-4VSB data segments into this TDM signal.

The FIG. 10 routine is also applicable to the FIG. 6 DTV transmitter.FIG. 12 lists the steps in a subsequent routine for validating that theoperation of legacy receivers will not be disrupted by the insertion ofrestricted-alphabet segments of parity bytes for transverse R-S FECcoding into TDM signal that defines a data field before subsequentconvolutional interleaving and trellis coding. A segment slot counterthat counts segment slots from one to 312 in a data field and then rollsover back to one is also used in the FIG. 12 routine. The counttherefrom is reset to a number indicative of the segment slot in thedata field it is proposed to fill with the final data segmentdescriptive of a data packet of symbol codes selected from a restrictedalphabet. The data packet is R-S FEC coded using the second type oflateral (207, 187) R-S FEC coding, thereby modeling the projectedoperation of the lateral (207, 187) R-S FEC encoder 26 of second type.The resulting 207-byte “seed” data is re-sampled to halve its code ratein accordance with a particular type of alphabet restrictions, modelingthe projected operation of the re-sampler 10.

The initial data segment in the pair is subjected to (207, 187) R-S FECdecoding of first type to recover a data packet therefrom, therebymodeling projected operation of the lateral (207, 187) R-S FEC decoderin a legacy DTV receiver. If this data packet has a valid PID and itsTEI bit indicates no uncorrected byte error remaining therein, thetransport stream de-multiplexer of a legacy DTV receivers would fail todiscard the data packet. So, insertion of the pair of data segments inthe proposed segment slots of the data field is unacceptable.Accordingly, the FIG. 12 routine is begun again after incrementing thecount supplied from the segment slot counter.

However, if the data packet recovered from the (207, 187) R-S FECdecoding of the initial data segment of the pair has an invalid PID orits TEI bit indicates uncorrected byte error remaining therein, the FIG.12 routine continues. The initial data segment of the pair is subjectedto (207, 187) R-S FEC decoding of first type to recover a data packettherefrom, thereby modeling projected operation of the lateral (207,187) R-S FEC decoder in a legacy DTV receiver. If this data packet has avalid PID and its TEI bit indicates no uncorrected byte error remainingtherein, the transport stream de-multiplexer of a legacy DTV receiverswould fail to discard the data packet. So, insertion of the pair of datasegments in the proposed segment slots of the data field isunacceptable. Accordingly, the FIG. 12 routine is begun again afterincrementing the count from the segment slot counter. However, if thedata packet recovered from the (207, 187) R-S FEC decoding of theinitial data segment of the pair has an invalid PID or its TEI bitindicates uncorrected byte error remaining therein, insertion of thepair of data segments in the proposed segment slots of the data field isacceptable.

The FIG. 11 routine is also applicable to the FIG. 6 DTV transmittermodified per FIG. 4. FIG. 13 lists the steps in a subsequent routine forvalidating that the operation of legacy receivers will not be disruptedby the insertion of pseudo-2VSB segments of parity bytes for transverseR-S FEC coding into TDM signal at defines a data field before subsequentconvolutional interleaving and trellis coding. The steps of the FIG. 13routine are similar to those listed in the FIG. 12 routine, with thefollowing exceptions. The seed data segment is generated by performinglateral (207, 187) R-S FEC coding of third type, rather than secondtype, on the randomized data packet to be transmitted using pseudo-2VSBsymbols. The re-sampling steps halve code rate by immediately repeatingeach bit of the seed data segment, modeling the projected operation ofthe re-sampler 24.

The paths involved in transverse R-S FEC coding are of concern, thenature of these paths being a variable that affects results. A/53prescribes convolutional interleaving of transmitted DTV signals. Theeffects of the convolutional interleaving and de-interleaving on thetransverse R-S FEC coding paths have to be considered. It is preferablethat the bytes within each transverse R-S FEC code are successivelytransmitted at intervals no shorter than the 77.3 microsecond durationof a data segment. U.S. patent application Ser. No. 10/733,645 filed 12Dec. 2003 describes a method for assuring this.

1. A method of generating more robust symbol coding using symbolsselected from a full symbol alphabet separable into a plurality ofcomponent restricted symbol alphabets, each having greater Euclideandistance between its component symbols than said full symbol alphabethas, in which said method symbols of said more robust symbol coding aresuccessively selected from ones of said plurality of restricted symbolalphabets in accordance with a prescribed pattern that is independent ofpreviously selected symbols.
 2. The method of claim 1 wherein said morerobust symbol coding is incorporated within a data stream that is thenconvolutionally interleaved and is subsequently trellis coded, therebygenerating time-dependent trellis codes in which different sets ofsymbols are precluded at prescribed times so as to increase the Hammingdistances between possible trellis codes.
 3. The method of claim 1wherein said prescribed pattern is chosen such that the average power ofsaid more robust symbol coding is substantially the same as for symbolcoding in which symbols are randomly selected from said full symbolalphabet.
 4. A transmitter connected for transmitting a radio-frequencycarrier modulated with a modulating signal including trellis codinggenerated by the claim 2 method.
 5. A receiver for trellis decoding astream of symbols selected from a full symbol alphabet separable into aplurality of component restricted symbol alphabets, each having greaterEuclidean distance between its component symbols than said full symbolalphabet has, at least some of which symbols are selected from saidplurality of restricted symbol alphabets in accordance with a pattern soas to generate more robust symbol coding, said stream of symbols beingReed-Solomon forward-error-correction coded before being convolutionallyinterleaved and subsequently trellis coded to generate time-dependenttrellis codes in which different sets of symbol codes are precluded atprescribed times so as to increase the Hamming distances betweenpossible trellis codes, said receiver comprising: apparatus fordetermining which data segments in said stream of symbols before theirconvolutional interleaving contained symbols selected from saidplurality of restricted symbol alphabets in accordance with saidpattern, and for supplying indications of when those data segmentsoccur; a trellis decoder connected for trellis decoding said stream ofsymbols to generate a trellis decoding result; a de-interleaverconnected for de-interleaving said trellis decoding result to reproducesaid stream of symbols including said more robust symbol coding; atwo-data-segments-to-one data compressor connected for selectivelycompressing said stream of symbols including said more robust symbolcoding reproduced from said de-interleaver, said selective compressingbeing done responsive to said indications of the occurrence of datasegments in said stream of symbols that before their convolutionalinterleaving contained symbols selected from said plurality ofrestricted symbol alphabets in accordance with said pattern; andReed-Solomon decoding apparatus connected for receiving said stream ofsymbols after being selectively compressed by saidtwo-data-segments-to-one data compressor, said Reed-Solomon decodingapparatus of a type capable of decoding lateral Reed-Solomonforward-error-correction coding.
 6. The claim 5 receiver, wherein saidpattern of selecting symbols from said plurality of restricted symbolalphabets is a prescribed pattern that is independent of previouslyselected symbols.
 7. The claim 5 receiver, wherein said pattern ofselecting symbols from said plurality of restricted symbol alphabetsdepends upon previously selected symbols.
 8. The claim 5 receiver,wherein said trellis decoder is a Viterbi trellis decoder of a type forresponding to information concerning the nature of each byte in saidstream of symbols supplied thereto for determining which types ofsymbols are precluded within each byte, said receiver furthercomprising: mapper circuitry, responsive to said indications of whendata segments in said stream of symbols before their convolutionalinterleaving contained symbols selected from said plurality ofrestricted symbol alphabets in accordance with said pattern, forgenerating a map of the byte pattern in said stream of symbols beforetheir convolutional interleaving; and a convolutional interleaverresponsive to said map of the byte pattern in said stream of symbolsbefore their convolutional interleaving for generating a map of the bytepattern in said stream of symbols after their convolutionalinterleaving.
 9. The claim 8 receiver, wherein said pattern of selectingsymbols from said plurality of restricted symbol alphabets is aprescribed pattern that is independent of previously selected symbols.10. The claim 8 receiver, wherein said pattern of selecting symbols fromsaid plurality of restricted symbol alphabets depends upon previouslyselected symbols.
 11. The claim 8 receiver, wherein the specific type ofsaid Reed-Solomon decoding apparatus is also capable of decodingtransverse Reed-Solomon forward-error-correction coding.
 12. The claim11 receiver, wherein said pattern of selecting symbols from saidplurality of restricted symbol alphabets is a prescribed pattern that isindependent of previously selected symbols.
 13. The claim 11 receiver,wherein said pattern of selecting symbols from said plurality ofrestricted symbol alphabets depends un previously selected symbols. 14.A method of generating a symbol code composed of symbols manifested asdifferent levels of a plural-level electric signal, said methodcomprising the steps of: a) randomizing data packets, each having aprescribed first number of bytes therein; b) forward-error-correctioncoding each of said data packets to generate a respective lateralReed-Solomon coded data segment; (c) re-sampling each of at leastselected ones of said lateral Reed-Solomon coded data segments byappending to each bit thereof a respective additional bit, some of saidadditional bits being ZEROes and the rest being ONEs in accordance witha prescribed pattern dependent on the position said additional bits willoccupy in data fields and independent of said bits said additional bitsare appended to, thereby generating a respective plurality ofreduced-code-rate data segments from each said selected lateralReed-Solomon coded data segment; (d) assembling said data fields so asto time-division-multiplex said lateral Reed-Solomon coded datasegments, each of said lateral Reed-Solomon coded data segments selectedfor re-sampling being replaced in said time-division multiplex by saidplurality of reduced-code-rate data segments generated by itsre-sampling, (e) convolutionally interleaving the bytes of data segmentsof said data fields to generate successive segments of convolutionallyinterleaved data fields; (f) trellis coding said successive segments ofconvolutionally interleaved data fields to generate a trellis code; (g)mapping successive nibbles of said trellis code into respective symbolsof said symbol code; and (h) inserting synchronizing signals into saidsymbol code.
 15. The method of claim 14, wherein said prescribed patternof additional bits is such that the average power in said symbol codegenerated from said reduced-code-rate data segments is substantially thesame as in symbol code generated from lateral Reed-Solomon coded datasegments that are not re-sampled to reduce code rate therein.
 16. Atransmitter connected for transmitting radio-frequency carrier-modulatedwith a modulating signal generated by the claim 14 method, which saidmodulating signal is composed of said symbol code having saidsynchronizing signals inserted therewithin in step (h) of said claim 14method.
 17. A transmitter connected for transmitting radio-frequencycarrier vestigial-sideband signal modulated in accordance with amodulation signal generated by the claim 14 method, wherein saidmodulating signal is composed of said symbol code having saidsynchronizing signals inserted therewithin in step (h) of said claim 14method, and wherein said reduced-code-rate data segments are of a sortthat give rise in said step (f) of trellis coding to symbol codes thatare selected from a first subset of four symbols during a firstprescribed set of symbol epochs and that are selected from a secondsubset of four symbols during a second prescribed set of symbol epochs,the symbols in said first subset of four symbols all differing from eachof the symbols in said second subset of four symbols.
 18. A receiverconnected for receiving a radio-frequency carrier modulated with amodulating signal generated by the claim 14 method, which saidmodulating signal is composed of said symbol code having saidsynchronizing signals inserted therewithin per step (h) of said claim 14method, said receiver comprising: front-end circuitry connected forresponding to said radio-frequency carrier modulated with saidmodulating signal to supply an intermediate-frequency carrier modulatedwith said modulating signal; analog-to-digital conversion circuitryconnected for digitizing said intermediate-frequency carrier modulatedwith said modulating signal; a demodulator connected for reproducingsaid modulating signal by demodulating said intermediate-frequencycarrier modulated with said modulating signal; digital filtering forequalizing said modulating signal as reproduced by said demodulator,thereby generating an equalized reproduced modulating signal; apparatusfor determining which data segments in said stream of symbols have beenre-sampled to reduced code rate in accordance with said pattern beforetheir being convolutionally interleaved, and for supplying indicationsof when those reduced-code-rate data segments occur; a trellis decoderconnected for trellis decoding said stream of symbols to generate atrellis decoding result; a de-interleaver connected for de-interleavingsaid trellis decoding result to reproduce said stream of symbolsincluding said reduced-code-rate data segments; atwo-data-segments-to-one data compressor connected for selectivelycompressing said stream of symbols reproduced from said de-interleaver,said selective compressing being done responsive to said indications ofthe occurrence of reduced-code-rate data segments in said stream ofsymbols before being convolutionally interleaved; and Reed-Solomondecoding apparatus connected for receiving said stream of symbols afterbeing selectively compressed by said two-data-segments-to-one datacompressor, said Reed-Solomon decoding apparatus of a type capable ofdecoding lateral Reed-Solomon forward-error-correction coding.
 19. Theclaim 18 receiver, wherein said trellis decoder is a Viterbi trellisdecoder of a type for responding to information concerning the nature ofeach byte in said stream of symbols supplied thereto for determiningwhich types of symbols are precluded within each byte, said receiverfurther comprising: mapper circuitry, responsive to said indications ofwhen data segments in said stream of symbols before their convolutionalinterleaving contained symbols selected from said plurality ofrestricted symbol alphabets in accordance with said pattern, forgenerating a map of the byte pattern in said stream of symbols beforebeing convolutionally interleaved; and a convolutional interleaverresponsive to said map of the byte pattern in said stream of symbolsbefore their convolutional interleaving for generating a map of the bytepattern in said stream of symbols after being convolutionallyinterleaved.
 20. The claim 19 receiver, wherein the specific type ofsaid Reed-Solomon decoding apparatus is also capable of decodingtransverse Reed-Solomon forward-error-correction coding.